[alanint]

All acceptance marks visible would indicate the entire pistol is Erfurt. Someone incorrectly added the frame's serial number over the chamber to make it "match", (at least in their own mind). This is a tough call between purposely sanded or somehow weathered. It would have to have been left somewhere where heavy wind and sand action worked on it for years, or perhaps it tumbled in the surf for a couple of years prior to being found and put back into service, at which time the chamber number was added.
This looks much more like a primitive third world find, rather than a battlefield pickup.